Video: Silicate Minerals: Chemical Classifications & Examples

Silicate minerals make up the bulk of the earth's crust. All silicates are made of the same building block, the silicon-oxygen tetrahedron. This lesson describes the major silicate minerals including feldspars and quartz.

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Silicate minerals make up the bulk of the earth's crust. All silicates are made of the same building block, the silicon-oxygen tetrahedron. This lesson describes the major silicate minerals including feldspars and quartz.

Minerals

Scientists have discovered over 4,000 different minerals, and more are discovered every year. However, only a few dozen minerals are relatively abundant in the earth. That's a lot easier to wrap your mind around. Furthermore, only eight elements make up the bulk of these minerals. The percent of these elements in the earth's crust is illustrated below. As you can see, oxygen and silicon are the most abundant elements in the earth's crust.

The elements that make up the crust of Earth

These two elements combine to form the most common mineral group, the silicates, accounting for more than 90 percent of the earth's crust. This lesson will describe the major silicate minerals, including feldspars and quartz.

Silicate Minerals

So, we've established that silicate minerals are the most abundant, and they contain silicon and oxygen. What else do we need to know about the silicates? Well, all silicates have the same fundamental building block, and that is the silicon-oxygen tetrahedron. That's a mouthful!

Silicon-oxygen tetrahedron

The silicon-oxygen tetrahedron forms a negative ion, containing four oxygen atoms surrounding a smaller silicon atom, as shown above. It has a molecular formula of (SiO4)4-, all of that with a negative 4 charge. Silicate minerals are composed of millions of these building blocks held together in various ways.

Some tetrahedra are arranged independently within the mineral, such as the olivine group of silicates. Amphiboles and pyroxenes are arranged as long chains of tetrahedra. Micas are arranged in sheets of tetrahedra. Now, last and certainly not least, feldspars and quartz are formed by three-dimensional arrangements of the tetrahedra.

The groups of tetrahedra are held together by positively charged ions; that is cations, such as iron, magnesium, potassium, sodium and calcium. As we will discuss briefly, silicates can be identified by the arrangement of the tetrahedra and the cation present within the mineral.

Most silicates are formed as molten rock cools and crystallizes. The conditions and the environment during which the cooling occurs will determine the type of silicate formed. Some silicates, for example, quartz, are formed near the surface of the earth, where there is low temperature and low pressure. Other silicates, such as olivine, form deeper, under conditions of higher temperature and higher pressure.

The chemical composition of the molten rock helps to determine the formed mineral as well; that is, the environment. For example, molten rock containing magnesium and iron can form minerals of the olivine group, while quartz is formed from molten rock composed of only silicon and oxygen, the silicon-oxygen tetrahedra, that is.

Feldspar and Quartz

Feldspars are by far the most common silicate mineral, making up more than half of the earth's crust. Feldspars are distinguished by their three-dimensional arrangement of the tetrahedra held together by various different cations. They are named for the cation present; for example, potassium feldspar contains potassium.

Quartz is the second most abundant mineral in the earth's crust, and it is the only common mineral made completely of silicon and oxygen. Now, like feldspar, quartz is comprised of a three-dimensional arrangement of the tetrahedra, but it lacks cations.

Other Silicates

While feldspars and quartz comprise the bulk of the silicates, and thus the bulk of the earth's crust, other less-common silicates are important. For example, micas, such as muscovite, are characterized by an arrangement of the tetrahedra in sheets. Therefore, micas can be cleaved into thin sheets of material, as the bonds holding the groups of tetrahedra together are not as strong as the bonds holding the silicon and the oxygen together within a tetrahedron. Amphiboles, such as hornblende found in granite, are formed from chains of tetrahedra. Now, the olivine group, as previously mentioned, is composed of millions of independent tetrahedra.

Lesson Summary

In summary, most minerals making up the surface of the earth are silicates, and they're composed of the building block we refer to as the silicon-oxygen tetrahedron (SiO4)4-. The environment and conditions under which molten rock crystallizes will determine the type of silicate formed. Silicates can form as independent tetrahedra, chained tetrahedra, sheeted tetrahedra and, finally, three-dimensional tetrahedra.

The arrangements of the tetrahedra are held together by cations such as potassium, iron, calcium and magnesium. Your feldspars and quartz are the most abundant silicates, comprising 75% of the earth's crust. Finally, less abundant silicates of importance include micas, amphiboles and the olivine group.

Learning Outcomes

Following this lesson, you will have the ability to:

Identify the most common minerals of the earth's crust

Describe the structure and importance of the silicon-oxygen tetrahedra

Explain how these tetrahedra are held together

List the two most abundant silicates, as well as some less abundant ones

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